1. Field of Invention
The present invention relates to a planographic printing plate, and particularly to a planographic printing plate having a heat sensitive recording layer of heat-mode type, wherein recording is performed using heat generated by means of photothermal conversion of a laser beam into heat.
2. Description of the Related Art
Recently, in accordance with progress in image recording technology, much attention is being directed to a technique in which a printing plate is formed directly without using a film original. In this technique a printing plate is scanned by a finely focused laser beam and characters and images are directly formed on the plate.
One of the examples of image recording materials in this field is what is called thermal type positive planographic printing plate. In this type of plate, an infrared absorbing agent in the photosensitive layer, by photothermal conversion action, generates heat by exposure and the generated heat causes exposed portion of the photosensitive layer to become soluble, resulting in formation of a positive image. Another example is a thermal type negative planographic printing plate. In this type of plate, firstly, the heat generated causes an acid to be formed by an acid generator or a radical to be formed by a radical generator. The acid or radical generated accelerates a radical polymerization reaction or an acid cross-linking reaction thereby forming insoluble portions and resulting in formation of negative images.
In the thermal type image recording process of this kind, photothermal conversion materials in the photosensitive layer generate heat due to laser light irradiation. The heat generated causes an image recording reaction. However on a roughened surface of an aluminum support with anodic oxide coating, because the thermal conductivity of the support is extremely high compared to that of the photosensitive layer, the heat generated in the vicinity of the interface of the support of the photosensitive layer diffuses to the inside of the support before being sufficiently used for image recording. Consequently, the decomposition reaction of positive photosensitive layer at the interface of the support of the photosensitive layer is insufficient. Consequently a residual film is formed in portions that should have been non-image portion causing the fundamental e problem of low sensitivity.
In order to deal with this problem, a variety of basecoats are being studied in order to improve developing property of the photosensitive layer in the vicinity of the interface of the support in positive type photosensitive layers. Also, for negative layers some experiments are being carried out in which the exposed parts are forced to be hardened by after-heating, or in which the surface of the layer is hardened easily by adjusting the photosensitive layer. Neither of the above mentioned experimentation was able to obtain satisfactory result.
Other methods being tried include using organic materials having low thermal conductivity for the support or the intermediate layer. However in this case, although insulative property is improved, the support is not appropriate for practical usage because adhesion with the recording layer is poor a non image portion of the layer has poor hydrophilic property.
Particularly in the recent market, there has been a strong demand to cut down exposure time in order to improve productivity and to extend the life of a laser by use at as low a power as possible. A planographic printing plate has been required which can be made directly using a laser beam; which uses the generated heat efficiently for an image recording reaction, whose non-image portion has excellent solubility in an acrylic developing solution; which has high sensitivity; and wherein a non image portion is not stained due to residual film formation.
The object of the present invention, therefore, is to provide a thermal type planographic printing plate which is writable due to infrared laser beam exposure, has high sensitivity and provides high quality printed matter which does not have stains.